1. Field of the Invention
The present invention relates to a power supply device that converts an input voltage and outputs a stabilized voltage, and in particular to the one equipped with a multiphase chopper.
2. Description of the Related Art
A switched-mode DC-DC converter has been disclosed in Patent Document 1 as a power supply device of this kind, which includes a plurality of switching circuits, each for on/off-controlling an input current supplied from a common input power supply; a smoothing circuit for synthesizing and smoothing the on/off-controlled currents by each switching circuit, so as to supply the resultant current to an electrical load; and a multiphase PWM control circuit for on/off-operating the plurality of switching circuits with the same period but out of phase with each other and feedback-controlling the on-duration for each switching circuit, so that an output voltage from the smoothing circuit becomes a predetermined target value.
This power supply device takes on/off control of switching elements constituting the switching circuits described above, based on instructions from a microcomputer for controlling equipment states, and performs voltage conversion.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-44941
The foregoing power supply device, by using the multiphase configuration, reduces amperage per phase as well as output ripples; however, if an open-circuit failure occurs in a switching element (for example, a MOSFET) of one phase, no current flows through the phase to which the switching element is connected; therefore, currents flowing through other remaining phases increase. If a current continues flowing through a switching element exceeding its allowable current, the element would be burnt out, in a worst-case scenario, leading to a fire. Moreover, even if not leading to a fire, if all of the phases fail, the power supply device sometimes cannot output power.
In a motor vehicle, etc., its battery is in some cases charged from a generator using the power supply device; therefore if the power supply device cannot output power, the battery cannot be charged, causing a problem in that the vehicle would eventually stop.